The relationship between the optical flow in the image plane and the velocities of objects in the three dimensional world is not necessarily obvious. We perceive motion when a changing picture is projected onto a stationary screen, for example. Conversely, a moving object may give rise to a constant brightness pattern. Consider, for example, a uniform sphere which exhibits shading because its surface elements are oriented in many different directions. Yet,when it is rotated, the optical flow is zero at all points in the image, since the shading does not move with the surface. Also, specular reflections move with a velocity characteristic of the virtual image, not the surface in which light is reflected."